1,721,021 research outputs found
Effect of thermo-mechanical parameters on the mechanical properties of Eurofer97 steel for nuclear applications
No full textEurofer97 steel has been recognised in Europe as the reference steel for nuclear application under high radiation density. Following to this a detailed knowledge of microstructure evolution is required for such steel after thermo-mechanical processing. In this paper the effect of thermo-mechanical parameters on the mechanical behavior of Eurofer97 was investigated by hot rolling and heat treatment on pilot scale. Results show a strong effect was found of reheating temperature before rolling on the material hardness, due to an increase of hardenability following the austenite grain growth. A minor effect was found of the hot reduction and the tempering temperature in the total investigated deformation range. A loss of impact energy was found coupled with the hardness increase. © 2018 Andrea Di Schino et al., published by De Gruyter
Analysis of nanoprecipitation effect on toughness behavior in warm worked AA7050 alloy
Full text linkCommonly adopted main methods aimed to improve the strength–toughness combination of high strength aluminum alloys are based on a standard process. Such a process includes alloy solution heat treatment, water-quench and reheating at controlled temperature for ageing holding times. Some alloys request an intermediate cold working hardening step before ageing for an optimum strength result. Recently a warm working step has been proposed and applied. This replaces the cold working after solution treatment and quenching and before the final ageing treatment. Such an alternative process proved to be very effective in improving strength–toughness behavior of 7XXX aluminum alloys. In this paper the precipitation state following this promising process is analyzed and compared to that of the standard route. The results put in evidence the differences in nanoprecipitation densities that are claimed to be responsible for strength and toughness improved properties
Production of TiA14V + SiC fibers composite and its structural evolution after heat treatments [Preparazione del composito Ti6Al4V+SiC fibre e sua evoluzione strutturale dopo trattamenti termici]
No full textA composite of Ti6A14V alloy reinforced with unidirectional SCS-6 SiC fibers has been fabricated by Hot Isostatic Pressing (HIP) at CSM (Centro Sviluppo Materiali). Fig.1 shows the inner structure of a SCS-6 fiber (left) and the composite cross-section (right). The stages of the preparation route are illustrated in fig.2: 1- preparation of the multilayered structure (5 layers of Ti6Al4V, 4 layers of SiC); 2- material cutting in its final shape (200 x 450 mm2); 3-material allocation in the mould, which is then evacuated (3x.10-6 mbar); 4- heating (T =890°C); 5- pressure application (Pmax= 1200 bar) for 30 minutes; 6- cooling down to room temperature; 7- composite extraction from the mould. The material has been studied by X-ray diffraction (XRD) and Atomic Force Microscopy (AFM.) XRD patterns of the composite and of the matrix alloy (powder) have been recorded at different temperatures up to 600°C in argon atmosphere and at room temperature after the thermal cycle for investigating the material structural evolution. Moreover, XRD spectra of composite and alloy were recorded at room temperature after heat treatments of 1 hour at 100, 200, 300, 400, 500 and 600°C. The experiments have been performed by employing a hightemperature X-ray camera. A sketch of its heating system is shown in fig.3. Overall spectra (5° ÷ 45° 2_ angular range, steps of 0.05° and counting time of 2s/step) and precision peak profiles of the {100}, {002}, {101}, {102}, {110} and {103} reflections (steps of 0.005° and 10s/step) were collected using Mo-Kα radiation (λ=0.071 nm). The central peak positions have been determined by an iterative procedure. The strain (εhkl) affecting {hkl} planes was obtained by eq.(1), where d and d0 are the interplanar spacings of the composite and of a stress free sample (Ti6Al4V powder) respectively. Fig.4 shows precision profiles of the first three diffraction peaks of matrix alloy and composite, collected at increasing temperatures up to 600°C. In both cases the peaks shift to lower angles as temperature increases; interplanar spacings are reported in table 1. Fig 5 shows the hexagonal cell parameters (a,c) vs temperature of composite and powder. Composite values are smaller than the powder ones for every temperature. The εhkl trends vs. temperature are plotted in fig.6: higher the temperature higher the strain, the values are negative and different for each set of planes. Fig. 7 compares the spectra of composite and powder before and after the thermal cycle. The composite peaks result shifted towards higher angles whereas those of the powder in the opposite way. Angular peak positions and interplanar spacings of composite in as-prepared condition (I) and after thermal cycle up to 600° C (F) are reported in table 2. The XRD spectra of composite after heat treatments of 1 hour at different temperatures are shown fig.8: the peak positions progressively shift towards higher angles by increasing the treatment temperature. Metal-fiber interface in the as-prepared material and after heating of 1 hour at 600°C in air has been investigated by AFM (see micrographs in figg. 9 and 10). The AFM observations show that the interface is not seriously damaged by the treatment in air. The increase of cell parameters during heating in composite and alloy is due both to thermal expansion and gas absorption. After cooling the increase of a and c observed in the alloy with respect the original values is due to trappig of gas, in particular nitrogen and oxygen, in the lattice: gas solubility increases at higher temperature and a residual part remains in the lattice after cooling. On the contrary, composite lattice was found compressed after cooling: X-ray diffraction peaks shift towards higher angles. The composite exhibits the same behaviour after heating of 1 hour at temperatures ranging from 100 to 600°C. Being σz=0, the compression along z-axis, giving the strain εz measured by XRD, is due to a biaxial state of tensile stresses in the plane of the sample surface (see fig.11). The origin of residual stresses of different sign in alloy and composite is connected to the role played by the fibers, which constrain the lattice expansion following gas absorption. This explanation is supported by AFM showing the integrity of metal-fiber interface also after the heat treatment at 600°C in air. Another results is that different strains affect different sets of crystalline planes of the composite. Therefore, to minimize the residual state of stress following heat treatments in atmosphere containing N 2 and O2, texture control seems of the utmost importance
Caratterizzazione meccanica del composito Ti6Al4V/SiCf dopo prolungata esposizione ad alta temperatura
No full textIl composito con matrice Ti6Al4V rinforzato con fibre lunghe di SiC (tipo SCS-6) è un materiale di grande interesse per applicazioni nei motori aeronautici. Non esistendo dati specifici di letteratura relativi al comportamento meccanico del composito dopo esposizione ad alta temperatura, simulante quella di esercizio, si sono eseguite prove meccaniche (trazione, microdurezza e indentazione strumentata con punta cilindrica a testa piatta-FIMEC) per studiare la presenza di un eventuale degrado rispetto al materiale non trattato.
Preliminarmente, per valutare eventuali effetti dell’atmosfera durante l’esposizione in temperatura, sono state eseguite prove FIMEC su campioni non protetti trattati per 1 ora a 100, 200, 300, 400, 500 e 600 °C in vuoto, argon e aria. I risultati mostrano chiaramente un peggioramento del comportamento meccanico rispetto al materiale non trattato. Tale degrado è di entità maggiore nel caso del trattamento in aria, tuttavia anche negli altri due casi la presenza residua di ossigeno comporta danni alla struttura del composito e ne compromette le caratteristiche meccaniche. Pertanto il materiale potrà esser posto in esercizio solo proteggendo preventivamente le fibre e l’interfaccia fibra-matrice.
Tutte le prove successive sono state eseguite dopo aver ricoperto per spruzzatura i provini con nitruro di boro. I trattamenti termici sono stati eseguiti in vuoto alle temperature di 400°C e 600°C con tempi di permanenza di 100, 500 e 1000 ore. Una serie di campioni è stata esposta per 100 e 1000 ore a 600 °C in atmosfera di gas inerte con tensione applicata di 15 MPa.
Le prove di trazione su questi provini sono state eseguite a temperatura ambiente e a 600 °C. La modesta variazione delle proprietà meccaniche rispetto a quelle iniziali, anche nelle condizioni più severe di trattamento termico, indica che non c’è degrado significativo della struttura, in particolare dell’interfaccia fibra-matrice. Questo è stato confermato anche da osservazioni SEM delle superfici di frattura.
Il lavoro dimostra che il composito Ti6Al4V/SiCf , una volta protetto con nitruro di boro, offre ottime garanzie per operare anche per tempi prolungati alle temperature di esercizio tipiche dei compressori dei motori aeronautici
Lattice expansion of Ti–6Al–4V by nitrogen and oxygen absorption
Full text linkHigh temperature X-ray diffraction (HT-XRD) measurements have been performed on Ti-6Al-4V alloy to investigate the lattice changes induced by oxygen and nitrogen absorption. Experiments have been performed on stress-free powder for its high surface-volume ratio and to avoid possible effects on XRD patterns due to texture change and recovery of defective structures induced by measurement cycle in temperature.
Cell parameters a and c, measured at increasing temperatures up to 600°C, show linear trends with different slopes. As temperature increases cell volume expands and c/a ratio changes. This is due to both lattice thermal expansion and absorption of oxygen and nitrogen. Part of gas remains entrapped in the metal after cooling to room temperature causing a residual lattice distortion. Results have been compared with literature data obtained in analogous tests performed on bulk material both in vacuum and inert gas atmosphere
Long-term heat treatments on Ti6Al4V-SiCf Composite. Part I - Microstructural characterization
No full textThe microstructure of Ti6Al4V-SiCf composite, in as-fabricated condition and after long-term heat treatments (up to 1,000 hours) in the temperature range 400 - 600 degrees C, has been investigated by means of high-temperature X-ray diffraction (HT-XRD), energy dispersion spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Particular attention was paid to the strains, arising during heating, and to the micro-chemical evolution of fibre-matrix interface. Micro-chemical examinations evidenced that a thin TiC layer has formed between the fibre carbon coating and the matrix during the fabrication process. TiC slows down further diffusion of carbon towards the matrix and guarantees the interface stability also for the most severe treatments examined here
Fabbricazione di compositi a matrice di titanio e fibre continue mediante plasma spray in alta pressione
No full textMaterials for high temperature liquid lead storage for concentrated solar power (Csp) air tower systems
Full text linkToday the technical limit for solar towers is represented by the temperature that can be reached with current accumulation and exchange fluids (molten salts are generally adopted and the max temperatures are generally below 600◦ C), even if other solutions have been suggested that reach 800◦ C. An innovative solution based on liquid lead has been proposed in an ongoing experimental project named Nextower. The Nextower project aims to improve current technologies of the solar sector by transferring experience, originally consolidated in the field of nuclear plants, to accumulate heat at higher temperatures (T = 850–900◦ C) through the use of liquid lead heat exchangers. The adoption of molten lead as a heat exchange fluid poses important criticalities of both corrosion and creep resistance, due to the temperatures and structural stresses reached during service. Liquid lead corrosion issues and solutions in addition to creep-resistant material selection are discussed. The experimental activities focused on technical solutions adopted to overcome these problems in terms of the selected materials and technologies. Corrosion laboratory tests have been designed in order to verify if structural 800H steel coated with 6 mm of FeCrAl alloy layers are able to resist the liquid lead attack up to 900◦ C and for 1000 h or more. The metallographic results were obtained by mean of scanning electron microscopy with an energy dispersive microprobe confirm that the 800H steel shows no sign of corrosion after the completion of the tests
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